Micronutrients including zinc, boron, copper, iron, chlorine, molybdenum and manganese are essential to plant growth but are only required in very small quantities. Micronutrients may be incorporated into fertiliser formulations but there are a number of challenges associated with effective incorporation of the micronutrient. Incorporation of the micronutrient into a bulk fertiliser can lead to a low solubility of the micronutrient such that it is not readily available to plants once applied to the soil. Coating a micronutrient onto a fertiliser can lead to micronutrient loss through abrasion. A bulk blended fertiliser, composed of a small proportion of micronutrient pellets or granules blended within one or more fertilisers, can lead to poor spatial distribution of micronutrient, giving an uneven application of micronutrient to the soil.
U.S. Pat. No. 6,322,607 addresses the problem of providing a zinc-enriched fertiliser that provides relatively efficient zinc uptake in soil or plants. It discloses a process for preparing zinc-enriched ammonium phosphate fertilisers wherein solid ammonium phosphate and a source of zinc such as zinc oxide or zinc sulphate are co-granulated. It is suggested that the process avoids or minimises reaction between the ammonium phosphate matrix and the zinc source and thereby provides a co-granulate wherein the zinc is more readily available for uptake in soil and plants.
The present inventors have sought to provide a process for preparing a micronutrient-enriched phosphate-based fertiliser wherein the micronutrient is readily available to plants.